On 06/01/2012 17:18, Neil wrote:
If you have a container and you put two vertical parallel wires in it,
why can't you determine the quantiy of a liquid poured into that
container based on the resistance measured between the two wires?

Only works if the resistance is in the right range. If the water is
impure then it conducts too well and looks like a dead short. Tiny
changes in the (im)purity of the water would affect the "level". That is
dipping your hand in for instance providing more ionic salts.

There is a trick using a central wire and an outer metal tube as a
variable capacitor with the central wire insulated that exploits the
difference in relative permitivity between the liquid and air. Sort of
an electronic version of the old manometer type oil tank measure.

These days most systems use an ultrasonic rangefinder to determine
distance to liquid surface. They seem to last ages (unless the guy who
delivers fuel sometimes replaces the battery unknown to me).

Regards,
Martin Brown